the tsunami in japan killed thousands. in some parts of some towns, there's no remaining evidence that anyone ever lived there. and now tonight the crisis has taken yet another turn, and we are covering a full-blown nuclear scare in japan. there are 17 nuclear power plants across japan, 54 nuclear reactors, but one plant in particular is in trouble. it's the fukushima plant, and if you've seen the pictures of it over this past weekend, there was one explosion in one building on saturday, another just yesterday and now a third reactor is in trouble at that same facility. japanese authorities are reporting tonight they have heard yet another explosion there within the last few hours. it's an urgent situation. the nuclear danger, the desperate human toll that goes on, and our team is in place to cover it. we will begin with our chief science correspondent, robert bazell, on the immediate nuclear danger there. bob, good evening. >>> brian, in what could have been the third explosion in four days, tensions are mounting in the s